It is a well known fact that in the operation of a furnace, using any ordinary fuel, the heat loss through the flue is considerable. There have been a number of concepts conceived to reduce these heat losses. Many of these concepts in one way or another rely on closing a damper in the flue after the fuel stops burning and the gases of combustion have been exhausted thereby trapping replacement air around the heat exchanger so that air, steam or hot water passing through the heat exchanger will continue to be heated or extract heat from the trapped hot replacement air. Some other concepts have employed heat exchangers in the flue stack so that heat can be extracted from the flue gases even when the fuel is burning. In at least one system in the prior art there is provided a valve controlled opening, out of the preflue chamber, so that when on fuel is being supplied for burning, the hot replacement air from a previous period of burning will be permitted to pass into the furnace room to heat the same.
The present system does not rely on closing the flue stack by a damper or using a heat exchanger in the flue, both of which arrangements are costly and in the case of closing the damper there is some danger involved if the damper jams or sticks as will be explained later. The present system forceably intercepts hot air which otherwise would pass through the flue and redirects that hot air, only during a safe period, into the overall heating arrangement.